PersistenceDescriptorUtils.java

import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.transform.OutputKeys;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import java.io.IOException;
import java.io.StringWriter;

public class PersistenceDescriptorUtils {

    private static final String JAVAX_PERSISTENCE_SCHEMA_GENERATION_DATABASE_ACTION = "javax.persistence.schema-generation.database.action";

    public static String setSchemaGenerationDatabaseAction(InputSource is, String action) throws ParserConfigurationException, IOException, SAXException, XPathExpressionException, TransformerException {
        final DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        final DocumentBuilder builder = factory.newDocumentBuilder();
        final Document doc = builder.parse(is);

        final XPathFactory xPathFactory = XPathFactory.newInstance();
        final XPath xPath = xPathFactory.newXPath();

        final Node persistenceUnit = (Node) xPath.evaluate("/persistence/persistence-unit", doc, XPathConstants.NODE);
        if (persistenceUnit == null) {
            // no persistence unit - do nothing
            return toString(doc);
        }

        final Node properties = (Node) xPath.evaluate("properties", persistenceUnit, XPathConstants.NODE);
        if (properties == null) {
            // construct a properties element which contains a property element
            persistenceUnit.appendChild(createPropertiesElement(doc, action));
            return toString(doc);
        }

        final Node property = (Node) xPath.evaluate("property[@name='" + JAVAX_PERSISTENCE_SCHEMA_GENERATION_DATABASE_ACTION + "']", persistenceUnit, XPathConstants.NODE);
        if (property != null) {
            // delete the existing property element
            properties.removeChild(property);
        }

        // add a property element
        properties.appendChild(createPropertyElement(doc, action));
        return toString(doc);
    }

    private static Element createPropertiesElement(Document doc, String action) {
        final Element properties = doc.createElement("properties");
        properties.appendChild(createPropertyElement(doc, action));
        return properties;
    }

    private static Element createPropertyElement(Document doc, String action) {
        final Element property = doc.createElement("property");
        property.setAttribute("name", JAVAX_PERSISTENCE_SCHEMA_GENERATION_DATABASE_ACTION);
        property.setAttribute("value", action);
        return property;
    }

    private static String toString(Document doc) throws TransformerException, IOException {
        final TransformerFactory factory = TransformerFactory.newInstance();
        factory.setAttribute("indent-number", 4);
        final Transformer transformer = factory.newTransformer();
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");
        try (final StringWriter sw = new StringWriter()) {
            transformer.transform(new DOMSource(doc), new StreamResult(sw));
            return sw.toString();
        }
    }

    private PersistenceDescriptorUtils() {
    }
}

PersistenceDescriptorUtilsTest.java

import org.junit.Assert;
import org.junit.Test;
import org.xml.sax.InputSource;

import java.io.StringReader;

public class PersistenceDescriptorUtilsTest {
    private static final String expected = "<property name=\"javax.persistence.schema-generation.database.action\" value=\"drop-and-create\"/>";

    @Test
    public void modify() throws Exception {
        String s = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
                "<persistence xmlns=\"http://xmlns.jcp.org/xml/ns/persistence\" version=\"2.1\">\n" +
                "    <persistence-unit name=\"Model2PU\">\n" +
                "        <jta-data-source>java:jboss/datasources/Model2DS</jta-data-source>\n" +
                "        <properties>\n" +
                "            <property name=\"javax.persistence.schema-generation.database.action\" value=\"none\"/>\n" +
                "            <property name=\"someProp\" value=\"someValue\"/>\n" +
                "        </properties>\n" +
                "    </persistence-unit>\n" +
                "</persistence>";
        try (StringReader r = new StringReader(s)) {
            Assert.assertTrue(PersistenceDescriptorUtils.setSchemaGenerationDatabaseAction(new InputSource(r), "drop-and-create").contains(expected));
        }
    }

    @Test
    public void add1() throws Exception {
        String s = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
                "<persistence xmlns=\"http://xmlns.jcp.org/xml/ns/persistence\" version=\"2.1\">\n" +
                "    <persistence-unit name=\"Model2PU\">\n" +
                "        <jta-data-source>java:jboss/datasources/Model2DS</jta-data-source>\n" +
                "        <properties>\n" +
                "            <property name=\"someProp\" value=\"someValue\"/>\n" +
                "        </properties>\n" +
                "    </persistence-unit>\n" +
                "</persistence>";
        try (StringReader r = new StringReader(s)) {
            final String s1 = PersistenceDescriptorUtils.setSchemaGenerationDatabaseAction(new InputSource(r), "drop-and-create");
            System.out.println(s1);
            Assert.assertTrue(s1.contains(expected));
        }
    }

    @Test
    public void add2() throws Exception {
        String s = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
                "<persistence xmlns=\"http://xmlns.jcp.org/xml/ns/persistence\" version=\"2.1\">\n" +
                "    <persistence-unit name=\"Model2PU\">\n" +
                "        <jta-data-source>java:jboss/datasources/Model2DS</jta-data-source>\n" +
                "    </persistence-unit>\n" +
                "</persistence>";
        try (StringReader r = new StringReader(s)) {
            final String s1 = PersistenceDescriptorUtils.setSchemaGenerationDatabaseAction(new InputSource(r), "drop-and-create");
            System.out.println(s1);
            Assert.assertTrue(s1.contains(expected));
        }
    }
}